Polypeptide growth factors play a key role in regulating the development of multicellular organisms and in the processes of tissue maintenance and repair. (see Cross and Dexter (1991) Cell, Vol. 64, pp.271-280; and Aaronson (1991) Science, Vol. 254, pp. 1146-1153.) At the cellular level, growth factors are involved in regulating proliferation and the progressive aquisition of the differentiated phenotype. Growth factors are capable of stimulating cellular proliferation as well as inhibiting cellular proliferation and many growth factors have been found to be multifunctional (Sporn and Roberts (1988) Nature, Vol. 332, pp.217-219). The highly coordinated functions of growth factors is perhaps best exemplified in the development of the hematopoiectic cell system (Metcalf (1989) Nature, Vol. 339, pp.27-30) where a limited number of stem cells give rise to a larger population of developmental restricted progenitor cells. These progenitors cells are further stimulated to proliferate and differentiate into the mature lymphoid, erythroid and myeloid cells. A balance between cell types and numbers of cells must be maintained throughout the developmental cascade. This requires the concerted actions of growth factors which commit a cell (now developmentally restricted) along a particular cell lineage, of growth factors which stimulate the proliferation of committed cells, and finally, of growth factors which promote the differentiation of the committed cells and inhibit the proliferation of the mature, fully differentiated cells.
Tumor cells represent naturally occurring examples of cells where the processes that control cellular proliferation and differentiation have been uncoupled (Cross and Dexter (1991) Cell, Vol. 64, pp.271-280; Aaronson (1991) Science, Vol. 254, pp.1146-1153). The observation that many types of tumor cells secrete growth factors suggests that these factors can contribute to the tumorigenic process as well as normal cellular processes. Tumor cells have been found to secrete autocrine growth factors which stimulate the proliferation of the tumor cells themselves and paracrine growth factors which stimulate surrounding cells to secret factors promoting the proliferation of the tumor cells. Paracrine factors can also stimulate the surrounding cells to provide a cellular environment promoting the survival of the tumor cells. For example, many types of tumor cells secrete growth factors that recruit endothelial cells and stimulate their proliferation and differentiation resulting in a new vasculature supplying nutrients for the tumor cells (Liotta et al., (1991) Cell, Vol. 64, pp.327-336).